1. Field of the Invention
This invention relates to apparatus and methods for the separation of liquids of differing densities in production streams from underground wells. More particularly, the invention relates to the downhole hydrocyclonic separation of a production stream from an oil well or ground water cleanup well into two streams, a first stream enriched in oil relative to the production stream and a second stream depleted in oil relative to the production stream, and transportation of the first, oil-enriched, stream to the surface.
2. Background of the Related Art
Cyclones are compact, centrifugal separators with no moving parts, which separate components of a mixture according to the relative densities of the components. For example, cyclones may be used to separate solids, liquids, gases or of some combination of phases from other solids, liquid, gases or combination of phases. Hydrocyclones are widely used in both onshore and offshore oil production in above-ground applications such as bulk water knockout from produced fluids or de-oiling produced water prior to either water reinjection into a formation or water disposal to a disposal site. In these applications a plurality of hydrocyclones are typically mounted within a pressure vessel assembly. Such an assembly resembles a shell-and-tube heat exchanger, in that the hydrocyclones are mounted to tube sheets which are sandwiched between flanges in the pressure vessel. The complete pressure vessel assembly typically has a single inlet for the produced liquid stream, such as a mixture of oil and water, and a plurality of outlets for the separated liquid streams. The assembly has an outlet for the "clean water" stream, which is relatively depleted in oil as compared to the production liquids, and an outlet for the "dry oil" stream, which is relatively enriched in oil as compared to the produced liquids.
Hydrocyclone type separators are able to continuously separate a production liquid stream into a heavy phase and a light phase using centrifugal forces created when a production liquid stream is provided into the conically shaped cyclone at a high speed in a substantially tangential direction. The liquid swirls around the inside of the cyclone at a high speed to create a substantial centrifugal force on the liquid which forces the heavier fluids radially outwardly from the lighter fluids. Typically, hydrocyclones will be designed to provide a centrifugal force on the liquid that is much greater than the gravitational force on the liquid, perhaps even several hundredfold greater, such that the effects of gravity on the liquid are negligible. Under these conditions, the heavy phase liquid is forced to the outer wall of the cyclone, thereby forcing the light phase liquid toward the center of the cyclone.
Because the outer wall of the cyclone is conical thereby producing a constant tangential speed to be imparted to the fluid, the heavier phase along the wall of the cone will migrate towards the end with the smaller diameter (the tail) and the lighter phase in the center will be pushed by the heavier fluid towards the end with the larger diameter (the head). In downhole applications, the capacity of a hydrocyclone separator is limited primarily by the diameter (or radius) of the separator head. The efficiency of the separator, i.e., the separator's ability to separate a heavy phase and a light phase, is a function of the square of the liquid tangential speed divided by the separator radius. Therefore, an efficient separation requires a high tangential speed. If the radius of the separator is increased, then the speed must also be increased to maintain a given efficiency. If the formation pressure acting on the liquid is low, then the speed of the liquid entering the separator will be slow and the separator must be designed with a sufficiently small radius in order to obtain a desired separation efficiency.
Hydrocyclones, as they are employed in oil production and environmental cleanup applications, are designed foremost to remove oil from water, that is, to produce a clean water stream with as low a concentration of oil as practicable. The dry oil stream will typically contain about 50 percent water, by volume, and may contain more than 50 percent water. Hydrocyclones, in a single-stage configuration, cannot produce both a completely water-free oil stream and a completely oil-free water stream; the design performance must be biased towards either the "dry oil" stream or the "clean water" stream. A clean water stream is obtained at the expense of "wet oil." Conversely, a dry oil stream is obtained at the expense of oily water. Hydrocyclone designs that are exemplary of those in the art are described in British Patent Application GB-A-2248198, which is incorporated herein by reference for all purposes, and U.S. Pat. No. 4,237,006, which is incorporated herein by reference for all purposes. Multi-stage separator assemblies including multiple hydrocyclones arranged in series, such as taught by U.S. Pat. No. 4,738,779, incorporated herein by reference for all purposes, can achieve improved separation at the expense of increasing the pressure drop of the liquids moving through the multi-stage assembly.
Hydrocyclones are also useful for making a preliminary separation of oil from water in the production liquids produced downhole in an oil well prior to the production liquids being transported to the surface. This is of particular value in high water cut wells, with a high water content, where the production liquids may comprise about 70 percent, or more, water. Conventionally, this water must be transported above ground, at significant cost and then disposed of, at additional expense. Hydrocyclone assemblies designed for above-ground use however, are not suitable for downhole applications where the assembly must be disposed within the bore hole of an oil well. This is because conventional hydrocyclone assemblies of sufficient capacity exceed the size limitations imposed by the diameter of the well. Further, previous attempts to overcome these problems have resulted in additional complications.
For example, PCT International Application WO 94/13920 discloses a downhole separation apparatus in which one or more hydrocyclones are contained within an axially elongate tubular housing, with the inlet of each hydrocyclone extending through the wall of the housing and having an opening external of the housing. The separated dry oil and clean water streams from each hydrocyclone are transported from the housing by a relatively complex system of pipes. With this apparatus there must be sufficient clearance between the housing and the adjacent wall of the well casing to provide a flow annulus for transporting the production fluid to the hydrocyclone inlets. This limits the diameter of the hydrocyclone housing for a given size casing, and hence reduces the capacity of the separation apparatus.
In applications where the formation pressure of the producing formation is too low, pumps and associated pump driving equipment, are required to lift produced fluids to the surface or to assist in the reinjection of water. WO 94/13930, for example, discloses placing a pump on the clean water stream to assist in reinjection of the clean water into the formation. U.S. Pat. No. 5,296,153 discloses pumping the dry oil stream to the surface and the clean water stream to another formation. In applications where the formation pressure is sufficient to drive fluid recovery, pumps are typically disposed downstream from the separator on the water outlet line to assist in the reinjection of the water into a selected formation. FIG. 1 shows a representative configuration for a downhole hydrocyclone separator assembly which is driven by the formation pressure. The hydrocyclone separator 24 is disposed in a well bore 10 in fluid communication with one or more production perforations and the resulting production stream. A by pass line 28 is connected to the water outlet of the separator and is connected to a pump 31 driven by a motor 29. The production zone 55 is defined at its lower end by a packer 57. The production fluids enter the casing through the perforations 23 and fill the volume in which the separator is disposed. The production fluids then enter the separator where the oil is recovered lifted to the surface and the water is routed for disposal. Typically, the pump 31 assists in the transportation of the water to a disposal area or zone. In these applications, it has been found that hydrocyclone separators in certain instances do not operate as effectively as in the applications where a pump is located upstream from the separator. It is believed that this reduction in effectiveness is attributable to the fact that the pump acts secondarily as a pre-input mixer to deliver a homogeneous mixture of oil, gas and water into the separator. However, many pumps provide mixing that is so vigorous that an oil-water emulsion is formed. The difficulties associated with breaking emulsion is well-documented. Therefore, while an upstream pump may increase the overall separator efficiency, it may also create an emulsion problem.
Therefore, there is a need for a separator assembly and methods that operates efficiently to separate oil and water. It would be desirable if the separator assembly provided the separator with a more homogeneous mixture of oil and water (and gas) than that of the production fluid without forming emulsions.